U.S. Pat. No. 3,757,780 discloses a needle assembly for use with a syringe or other type of medical injector, the needle assembly including a movable filter element which shifts axially within the hub of the needle depending on the direction of liquid flow therethrough. To withstand such movement and the forces generated by fluid flowing through the hub, particularly as such fluid is being drawn into the syringe, the filter element or disc must be formed of a relatively rigid unyielding material, thereby severely limiting the selection of materials suitable for use in a structure having a movable filter.
Other patents illustrating the state of the art are U.S. Pat. Nos. 3,757,779, 3,736,932, 3,722,697, 3,449,081, 1,539,314, 964,730, and 128,257.